1. Field of the Invention
The present invention relates to a method and an apparatus for effecting an interference fit of two parts, and more particularly to techniques for minimizing the amount of unnecessary deformation of the parts during an operation to effect the interference fit.
2. Discussion of the Prior Art
JP-U-63-144136 (laid-open publication of Japanese Utility Model Application NO. 62-35891) discloses conventional method and apparatus for effecting an interference fit of two mating parts of an assembly. In this conventional interference-fit method, at least one of the mating parts is forced onto the other part by a rigid pressure head, to thereby effect an interference fit for assembling the two parts. The conventional apparatus disclosed in the above publication includes (a) a first holder device for holding a first part which is one of the two parts to be assembled, (b) a second holder device for holding a second part which is the other of the two parts, such that the second part is movable toward the first part held by the first holder device, in the direction of the interference fit with the second part; and (c) a pushing device including a pressure head (equipped with a chuck if necessary) which is adapted to engage with the second part held by the second holder device and force the second part onto the first part held by the first holder device. Thus, the conventional technique for effecting an interference fit of the first and second parts uses a pressure head which directly forces the second part into the first part to achieve the interference fit of these two parts.
However, the conventional technique for effecting an interference fit suffers from the following problem.
In the case where the second part in the form of a shaft is axially forced or press-fitted into a hole axially formed through the first part, for example, this press fitting operation is possible provided only a mating portion of the second part which engages with the first part is forced in the direction of the interference fit, namely, in the axial direction of the second part. In the conventional technique indicated above, however, a mechanical force F produced by the pressure head during an press-fitting operation of the two parts is received by the second part at one of its opposite axial ends remote from the first part, as indicated in the upper part of FIG. 3. As a result, the axial force acting on the second part is evenly distributed over the entire axial length of the second part, as indicated in the lower part of FIG. 3. Consequently, the conventional technique causes a considerably large axial compressive stress acting on the portion of the second part other than the mating portion at which the interference fit is achieved.
The above phenomenon results in the following problem, where the mating portion of the second part is made of a hard material such as a steel. Usually, the axial compressive force acting on the second part during the press-fitting operation is not evenly distributed on the opposite sides of the axis of the second part, and tends to generate a moment that causes the second part to be bent with respect to its axis. Where the moment generated is large, the portion of the second part other than the mating portion press-fitted in the first part is likely to suffer from undesirable permanent deformation. If the portion of the second part other than the mating portion includes a region that can be easily bent due to a cutout or local reduction in the modulus of section, plastic deformation may take place due to stress concentration in that region, leading to permanent deformation of the second part.
The large axial compressive force acting on the portion of the second part other than the mating portion will cause the following problem, where the mating portion is made of an elastic material such as rubber. That is, the axial compressive force produced during the press-fitting operation will cause elastic deformation of the elastic portion of the second part in the direction perpendicular to the direction of the press fitting of the two parts. This elastic deformation results in an increase in the amount of interference between the elastic portion of the second part and the corresponding mating portion of the first part, whereby the press fitting is difficult to achieve, or likely to cause surface damaging or breakage of the elastic portion of the second part.
In the conventional technique for effecting an interference fit of two parts, a considerably large axial compressive force produced during the press-fitting operation unnecessarily acts on a portion of one of the two mating parts which is displaced relative to the other, so that the displaced part undesirably suffers from deformation at that portion.
Where the mating portion of the second part is made of an elastic material, that surface of this elastic portion is generally coated with a lubricant. However, when the amount of interference between the mating portions of the first and second parts is relatively large, the lubricant is wiped off by the first part, leading to an insufficient lubricating effect of the lubricant. In view of this drawback, the conventional method is adapted such that the second part is first forced into the hole of the first part by a distance larger than the nominal length of the interference fit of the two parts so as to ensure fitting of the mating portions of the two parts, and then the second part is displaced in the reverse direction so as to release the undesirable compressive stress in the elastic mating portion of the second part. Thus, the conventional method requires an extra step where the mating portion of the second part is made of an elastic material.